Phosphorus bioavailability following incorporation of green manure crops

Incorporating green manure crops into soil may increase P bioavailability for succeeding crops. We conducted a greenhouse study to evaluate the effects of green manures on biomass and P utilization of a succeeding grain sorghum [Sorghum bicolor (L.) Moench] crop. Four perennial forages and four winter annual cover crops were grown in pots, killed, and incorporated into the soil before planting sorghum in the same pots. Sorghum P uptake was positively correlated with perennial forage P uptake. Among winter cover crops, sorghum P uptake following white lupine (Lupinus albus L.) was lower than in all other treatments, including the control (no previous cover crop), even though lupine biomass, N content, and P uptake were two to three times greater than those of the other winter cover crops. Phosphorus uptake differed slightly among the other three winter cover crops but sorghum P uptake was not correlated to winter cover crop P uptake. Thus, among winter cover crops, plant type rather than P uptake seemed to influence the subsequent sorghum crop's P uptake. However, sorghum biomass following the three winter cover crops other than lupine was greater than sorghum biomass in the control treatment, indicating that there was a beneficial cover crop rotation effect among these three winter cover crops. Soil samples were collected and analyzed for Bray-1 P when green manure crops were planted, when they were incorporated into the soil, when sorghum was planted and when sorghum was harvested. These data showed that the Bray-1 soil P test has little potential to predict differences in P uptake and release among different types of green manures and it has limited potential to predict P uptake by sorghum following incorporation of green manures.